In recent years, there has been a tendency to reduce the weight of the bearing apparatus which rotatably supports the driving wheel relative to a suspension apparatus of the vehicle in order to improve the fuel consumption. Especially, the bearing apparatus for the driving wheels such as rear wheels of a rear wheel drive vehicle, front wheels of a front wheel drive vehicle and all wheels of a 4WD (4-wheel Drive) vehicle has been improved in its united structure to achieve the driving stability and high rigidity.
As shown in FIG. 19, the bearing apparatus for a prior art driving wheel of vehicle is formed such that a wheel hub 50, a double row rolling bearing 60 and an constant velocity universal joint 70 are assembled as a unit. One of inner raceway surfaces 51 of a double row rolling bearing is formed on the wheel hub 50 and the other of the inner raceway surfaces 72 is formed on an outer joint member 71 of the constant velocity universal joint 70. The wheel hub 50 is formed integrally with a wheel mounting flange 53 to mount a wheel (not shown) on one end of a cylindrical portion 52. Hub bolts 54 that secure the wheel are equidistantly arranged along the periphery of the flange 53. The inner raceway surface 51 is formed on the outer circumferential surface of the cylindrical portion 52 near the wheel mounting flange 53.
The constant velocity universal joint 70 comprises the outer joint member 71, a joint inner ring, a cage and a torque transmission balls (not shown). The outer joint member 71 has a cup shaped mouth portion 73, a shoulder 74 forming the bottom of the mouth portion 73, a stem portion 75 axially extending from the shoulder 74, axially extending curved track grooves 76 formed on the inner surface of the mouth portion 73 and an inner raceway surface 72 formed on the outer circumferential surface of the shoulder 74. The stem portion 75 is inserted into the cylindrical portion 52 of the wheel hub 50 with the end surface of the cylindrical portion 52 abutting the shoulder 74 of the outer joint member 71. The pitch between the inner raceway surfaces 51 and 72 and the internal clearance is defined by the axial positioning of the wheel hub 50 and the outer joint member 71, via the abutment therebetween mentioned above. The stem portion 75 is formed as a hollow configuration by a through bore 77 communicating with the mouth portion 73. An end plate 78 is arranged on the end of the through bore 77 at the side of the mouth portion 73 to prevent leaking of lubricating grease filled within the mouth portion 73.
The double row rolling bearing 60 comprises an outer member 61 and a double row rolling element 62. The outer member 61 has a flange 63 integrally formed therewith to be mounted on a body of vehicle (not shown) and double row outer raceway surfaces 64 and 64 formed on the inner surface thereof. The double row rolling elements 62 and 62 are arranged between the outer raceway surfaces 64 and 64 and the inner raceway surfaces 72 of the outer joint member 71 and rotatably held therein by cages 65 and 65. Seals 66 and 67 are arranged at the ends of the outer member 61 in order to prevent leaking of grease contained within the bearing and also to prevent ingress of rain water or dusts.
The wheel hub 50 is formed with hardened irregular portion 55 on the bore surface of the cylindrical portion 52. The wheel hub 50 and the outer joint member 71 are mutually connected via plastic deformation by radially outwardly expanding the bore of the stem portion 75 and by making the hardened irregular portion 55 to bite into the cylindrical portion 52. When carrying out the radial expanding operation by press work, it is carried out by initially inserting the stem portion 75 into the cylindrical portion 52 of the wheel hub 50. The wheel mounting flange 53 is laid on a pedestal 80. Finally, a caulking jig (punch) 81 is pressed into the through bore 77 of the stem portion 75. The caulking jig (punch) 81 has a large diameter portion 81a larger than the inner diameter of the through bore 77 (see Japanese Laid-Open Patent Publication No. 50846/2001).
In the bearing apparatus for a driving wheel, the load at the side of the wheel mounting flange 53 (i.e. the outboard side) is supported by the radially enlarged portion when the bending moment is applied to the apparatus during turning of the vehicle. Accordingly, the stem portion 75 including the radially enlarged portion is bent and thus the tension and compression stresses are alternately applied to the bore of the stem portion 75. In order to prevent the fatigue failure of the stem portion 75 which would be caused by these repeating stresses, it is necessary to increase the rigidity.
On the contrary, when trying to increase the rigidity of the stem portion 75 by increasing the thickness without changing the size of the apparatus, not only does the press work becomes difficult because of the reduction of the diameter of the through bore 77, but the improvement to the rigidity is limited because of the prevention of reduction of the weight of the apparatus.